In this picture you can see the hot lava erupting
out of the volcano. The black rock around the vent is hardened lava
from earlier eruptions.

What is it?

Volcanoes are mountains that come in various shapes and sizes, some towering
tens of thousands of feet high and others just small domes that rise about
100 feet. Unlike other mountains, though, these natural wonders can erupt
with powerful and potentially lethal force.

Openings, or vents, in the volcano lead below the earth's surface to
very hot chambers several miles underground where there are pools of magma
(molten or melted rock), pieces of hot solid rock, and hot gases. Shifts
in the earth can build up pressure beneath the volcano. The magma, which
is lighter than the surrounding solid rock, is forced upward by the pressure
and may ultimately break though, causing a volcanic eruption.

A non-explosive, or effusive, volcano produces lava (magma that has erupted
from the earth). An explosive volcano produces ashes. Explosions (without
lava) usually occur first because there are lots of gases inside the magma.
When you open a bottle of soda, bubbles (which are gas particles) escape
first, then liquid can flow; similarly, gas escapes from the magma and,
once the magma is flattened, the liquid lava can flow.

The earth's outermost layer, or crust, is made up of a number of shifting
tectonic plates. Most active volcanoes are located near the boundaries
between these plates. For instance, several plates border each other in
the Pacific Ocean Basin, an area where you'll find so many active volcanoes
that it's been named the "Ring of Fire." Among the plate boundary
volcanoes in this "Ring" is Washington's Mount St. Helens.

Volcanologists, scientists
who study volcanoes, believe Mount Rainier in Washington state is the
most dangerous U.S. volcano  it could erupt one day and destroy
areas 50 or more miles away.